1. Field of the Invention
This invention relates to a detection device for detecting venous pooling in a human body and to the use of this detection device in a therapy device for preventing vasovagal syncope, particularly to the prediction of neurally mediated bradycardia and hypotension.
2. Description of the Prior Art
The exact etiology of vasovagal syncope remains unknown, but parasympathetic and sympathetic activity have been found to participate in the pathogenesis of vasovagal syndrome. Particularly, vasodepressor reaction is caused by the Bezold-Jarisch reflex. There are three modes of vasovagal responses: cardioinhibitory, vasodepressor and mixed response. Accordingly, patients may have profound bradycardia and a major component of vasodilation as well. It has been postulated that vigorous contraction of relatively empty ventricle activates myocardial sensory receptors, which, in susceptible persons, initiates an inhibitory reflex that results in hypotension, bradycardia or both. Vasodepressor reactions are believed to be caused by activation of unmyelinated left ventricular vagal nerve endings known as C-fibers normally being excited by catecholamines, sympathetic nerve stimulation and left ventricular pressure. The most important physiologic finding is that spontaneous vasodepressor reactions often occur in the context of a sympathetic stimulation as a response to the venous pooling. Accordingly, every vasovagal syncope is preceded by the venous pooling, the sudden heart rate increase, and by the vigorous ventricular contractions.
Bradyarrhythmias associated with vasovagal syncope must be treated by permanent pacing. Vasovagal syncope occurs usually in the erect body position with gravitational stress on venous return. The first pacing mode of choice was VVI pacing with hysteresis. U.S. Pat. No. 4,856,523, describes the inclusion of the hysteresis feature in a rate-responsive pacemaker, in an attempt to prevent competition between the pacemaker and the heart's SA node, when the anterograde conduction path is restored. This patent proposes to vary the hysteresis rate as a function of the pacemaker sensor rate, to a predetermined level upon sensing of the natural heart contraction during the escape interval. U.S. Pat. No. 4,363,325 discloses a multiple-mode pacer which automatically switches from an atrial synchronous mode to a ventricular inhibited mode when the intrinsic atrial rate drops below a preset hysteresis rate. Nevertheless, dual chamber pacing assures maximal contribution of atrial contraction to the ventricular filling, maintaining the natural sequence of cardiac activation. The majority of patients suffering from malignant vasovagal syndrome have intact retrograde atrioventricular conduction, which may cause the pacemaker mediated tachycardia. DVI mode of pacing may provoke atrial arrhythmias due to the fact that the atrial pacing pulse occurs irrespective of the atrial spontaneous activity. The best mode of pacing, preventing these problems in pacing therapy, is DDI mode, especially in patients having intact retrograde conduction. That is the mode when a pacemaker acts like the two independent pacemakers inhibited by spontaneous activity (DDI=AAI+VVI). When there is no vasovagal attack, there is no need for cardiac pacing. It is known in the art that the patient's baseline resting rate can fall as low as 50 beats per minute, especially during the sleep. Moreover, DDI pacing rate being 20 beats per minute higher than resting heart rate may provide significant hemodynamic improvement and maintain consciousness during a vasovagal attack. Accordingly, DDI pacing with hysteresis is the most appropriate pacing mode. However, allowing the patient to have a normal low sinus rate during the night and at rest, as well as starting to pace with 20 beats per minute higher frequency than basic rate (for instance during the day) would require special function of a pacemaker.
Such a pacemaker is disclosed in the U.S. Pat. No. 5,284,491. It has a programmable lower rate and upper rate, a programmable lower hysteresis rate corresponding to a lower rate hysteresis interval, and a programmable intermediate rate, an upper pacing rate and a lower pacing rate. A microprocessor measures the average rate of change in the intervals between consecutive ventricular depolarizations, and compares the last intrinsic escape interval to the lower rate hysteresis interval. If the last intrinsic ventricular interval will be longer than the lower rate hysteresis interval, and if the value of average rate of change in the intervals between consecutive ventricular depolarizations is greater than a first preselected value but less than a second preselected value, the pacemaker stimulates at the lower rate hysteresis and thereafter gradually increases the pacing rate up to the intermediate rate while the pulse generator is in the demand mode. A time counter maintains a continuous pacing at the intermediate rate for a predefined period of time, and the pacing rate is gradually decreased down to the lower pacing rate.
In a patient suffering from vasovagal attack and having such a pacemaker implanted, there is a late start of pacing therapy, when vagal reflex has been already initiated and bradycardia occurred. Such a system cannot assure to the patient a significant release of the symptoms. Moreover, cardioinhibitory response is associated with hypotensive reaction that already occurred when the pacing starts. The best mode of pacing would be with a system which initiates the sequential pacing with the rate higher than the resting rate, but prior to the bradyarrhythmic event. Such a mode of pacing would be possible only with the system comprising certain means for prediction of the vasovagal attack.
A kind of system comprising a prediction means for vasovagal syncope has been disclosed in JACC Vol. 25, No. 1, January 1995, page 70 D. G. Benditt et al. Characterization of Subcutaneous Microvascular Blood Flow During Tilt Table--Induced Neurally Mediated Syncope. It was demonstrated by means of the laser Doppler flowmeter, that a tilt-up test causes the subcutaneous blood flow reduction as well as the blood flow oscillations having a specific frequency.
There are several blood flow measurement methods being feasible to be utilized in implantable systems. There is a possibility to measure the impedance by means of the very low current strength, such as disclosed in EP-A-0 620 420. The impedance is modulated by the ionic liquid flow.
Blood flow parallel to the catheter can be measured by means of the system disclosed in U.S. Pat. No. 4,691,709. The catheter employs an elongated optical fibre member wherein the light transmission is modulated due to the pressure acting thereon. The flow velocity is determined as a function of the difference between the total pressure and the transversely acting pressure.
Our invention disclosed in U.S. Pat. No. 5,243,976, No. 5,316,001 and No. 5,318,595 is a novel principle of physiologic cardiac pacing based on the tricuspid blood flow measurement. EP-A-0 740 565 discloses the cardiac pacing system utilizing the blood flow through either right pulmonary veins or superior vena cava as a parameter for pacing control. Apart from the ultrasonic Doppler system, we have also developed the low-power blood flow measurement methods to be utilized in these inventions. The hydrodynamic sensor is disclosed in our U.S. Pat. No. 5,271,408. The ultrasonic system measuring the blood flow by means of an ultrasonic beam being perpendicular to the flow velocity vector is disclosed in EP-A-0 739 183. The galvanic flow sensor is disclosed in EP-A-0 752 826 being a polarizable electrode within the blood flow, having the concentration overvoltage modulated by the flow velocity. All these methods are feasible to be incorporated within the implantable therapy device such as a pacemaker.